This invention relates to pressure gauges and more particularly to the type of pressure gauge that uses a reactive electronic element as a transducer to convert a pressure differential to a voltage differential.
Pressure gauges for nuclear power plants must be capable of reliably operating over a wide range of severe environmental conditions of temperature, radiation, and seismic shock. One suitable gauge is described in co-pending patent application U.S. Ser. No. 273,871, filed Jan. 2, 1981, for "VARIABLE RELUCTANCE PRESSURE TRANSDUCER" wherein the transducer consists of two oppositely disposed housing blocks defining a sealed inner chamber in which is disposed a magnetically permeable diaphragm. Fluid is introduced into the chamber such that the differential pressure to be measured deflects the diaphragm. Mounted within the blocks on either side of the diaphragm are two inductance coils which are magnetically coupled to the diaphragm.
The sensor coils must not be exposed to the fluid in the chamber, and any cover between the coils and the chamber must not interfere with the magnetic coupling to the diaphragm. Therefore, the coil cover must be non-magnetic, although the preferred material of the blocks is magnetic. Additionally, the cover must be strong enough to resist overpressure of the fluid, have thermal expansion properties that are compatible with the block and coil mounting structure, and be immune to corrosion. The prior art as represented by the aformentioned copending application is not completely satisfactory in meeting these requirements. In one embodiment of said co-pending application, each block is made from non-magnetic stainless steel, and includes a solid, integral web of block material over the coil detector. While providing overpressure protection and sealing against the fluid, the non-magnetic block has different thermal expansion properties from the magnetic diaphragm. At higher temperatures, thermal stresses are imposed on the diaphragm resulting in erroneous pressure measurement. These thermal stresses cannot be compensated by electronic signal processing.
Another embodiment of the co-pending application includes use of a non-magnetic seal disk over the element welded to an opening in the magnetic block. The problems associated with the welded disk include potential for weld cracking due to differential thermal expansion, and long-term crevice corrosion at the disk and block interface. Also, since the disk must be recessed slightly from the chamber to allow room for a weld bead along its periphery, overpressurization of the diaphragm against the disk can produce permanent diaphragm deformation.